1. Field of the Invention
The present invention relates to liquid crystal displays (LCDs). More specifically, the present invention relates to vertical alignment LCDs, with very high contrast ratios.
2. Discussion of Related Art
Liquid crystal displays (LCDs), which were first used for simple monochrome displays, such as calculators and digital watches, have become the dominant display technology. LCDs are used routinely in place of cathode ray tubes (CRTs) for both computer displays and television displays. Various drawbacks of LCDs have been overcome to improve the quality of LCDs. For example, active matrix displays, which have largely replaced passive matrix displays, reduce ghosting and improve resolution, color gradation, viewing angle, contrast ratios, and response time as compared to passive matrix displays. Vertical alignment nematic LCDs address some of the drawbacks of conventional twisted nematic LCDs, such as low contrast ratio.
FIGS. 1A-1B illustrate the basic functionality of a pixel of a vertical alignment LCD 100. For clarity, the LCD of FIGS. 1A and 1B uses only a single domain. Furthermore, FIGS. 1A-1B are simplified for clarity and omit many processing layers. For example, between substrate 110 and electrode 120, actual displays would likely include various metal layers used for electrical connections as well as passivation layers (i.e. insulating layers) that separate the metal layers. In addition the LCD of FIGS. 1A-1B is described in terms of gray scale operation. Well known, conventional color techniques such as the use of color filters or field sequential coloring can be used to add colors.
For further clarity and consistency, the various components of the pixels and the displays in the figures are described from the perspective of the display being flat on a table and the reader being in front of the table. The perspective of the written description does not change whether the figures shows a slice of the display from the edge of the display such as FIGS. 1A and 1B or when an overhead view of a pixel or display is shown such as FIGS. 2. Thus, for figure with a view from the edge of the display, the two axes shown would be up/down axis and left/right axis. Suitable terms that are used to describe position relative to the up/down axis include “above”, “below”, “on top off”, and “underneath”. For the left/right axis suitable terms include “to the left of” and “to the right of”. For Figures with an overhead view, the two axes used are the left/right axis and front/back axis. The front/back would be like a north/south axis for a map on the table. Suitable terms that are used to describe placement relative to the front/back axis include “in front of” (which would be equivalent of to being “south of” on a map) and “in back of” (which would be equivalent to being “north of” on a map). Furthermore, as used herein the up/down axis is the vertical axis, the left/right axis is the horizontal dimension, and the front/back axis is the longitudinal axis.
LCD 100 has a first polarizer 105, a first substrate 110, a first electrode 120, a first liquid crystal alignment layer 125, liquid crystals 130, a second liquid crystal alignment layer 140, a second electrode 145, a second substrate 150, and a second polarizer 155. Specifically, polarizer 105 is attached to the bottom of substrate 110, first electrode 120 is formed on top of substrate 110, and first liquid crystal alignment layer 125 is formed over first electrode 120. Liquid crystals 130 are in between first liquid crystal alignment layer 125 and second liquid crystal alignment layer 140. Common electrode 145 is above liquid crystal alignment layer 140. Common electrode 145 is formed on the bottom of second substrate 150 and second polarizer 155 is attached to the top of substrate 150. Generally, first substrate 110 and second substrate 150 are made of a transparent glass. First electrode 120 and second electrode 145 are made of a transparent conductive material such as ITO (Indium Tin Oxide). First liquid crystal alignment layer 125 and second liquid crystal alignment layer 140, which are typically made of a polyimide (PI) layer, align liquid crystals 130 near a vertical resting state, thus liquid crystals 130 have a small pre-tilt angle from the vertical alignment. In operation, a light source (not shown) sends light from below first polarizer 105, which is attached to the bottom of first substrate 110. First polarizer 105 is generally oriented with polarization axis in a first direction and second polarizer 155, which is attached to the top of second substrate 150, is oriented with polarization axis that is perpendicular to first polarizer 105. Thus, light from the light source would not pass through both first polarizer 105 and second polarizer 155 unless the light polarization were to be rotated by 90 degrees between first polarizer 105 and second polarizer 155. For clarity, very few liquid crystals are shown. In actual displays, liquid crystals are rod like molecules, which are approximately 5 angstroms in diameter and 20-25 angstroms in length. Thus, there are over 5 million liquid crystal molecules in a pixel that is 80 μm width by 240 μm length by 3 μm height. Although not shown, many liquid crystal displays (particularly active matrix LCDs) include a passivation layer on bottom of first electrode 120. The passivation layer serves as an insulating layer between the first electrode 120 and devices and conductors that may be formed on the substrate 110. The passivation layer is commonly formed using silicon nitrides.
In FIG. 1A, liquid crystals 130 are vertically aligned with a pre-tilt angle. In the vertical alignment, liquid crystals 130 would not rotate light polarization from the light source. Thus, light from the light source would not pass through LCD 100 and gives a completely optical black state and a very high contrast ratio for all color and all cell gap. However due to the need of a pre-tilt angle (as explained below) there is some light leakage even when a dark pixel is desired. Thus, while conventional vertically aligned LCDs provide a big improvement on the contrast ratio over the conventional low contrast twisted nematic LCDs, even higher contrast ratios are desired for advanced LCD applications.
However, as illustrated in FIG. 1B, when an electric field is applied between first electrode 120 and second electrode 145, liquid crystals 130 reorientate to a tilted position. Liquid crystals in the tilted position rotate the polarization of the polarized light coming through first polarizer 105 by ninety degrees so that the light can then pass through second polarizer 155. The amount of tilting, which controls the amount of light passing through the LCD (i.e., brightness of the pixel), is proportional to the strength of the electric field. Generally, a single thin-film-transistor (TFT) is used for each pixel. However for color displays, a separate TFT is used for each color component (typically, Red, Green, and Blue).
As illustrated in FIG. 1B, for all the liquid crystals tilt in the same direction. Having all liquid crystals in a single domain tilt in the same direction increases the brightness of a display and therefore increases the contrast ratio. In conventional vertically aligned LCDs, the pre-tilt angle makes the liquid crystals tilt in the same direction. However, the pre-tilt angle also allows light to pass through the LCD even when the pixel is turned off. Typically, the liquid crystal alignment layers are made using a well-known rubbing technique. This rubbing technique is relatively expensive and does not allow fine control on the pre-tilt. Furthermore, the rubbing technique complicates the fabrication of advanced LCDs with a complex multi-domain structure, because the liquid crystal alignment layer over each separate domain has to be rubbed in a different direction. Hence there is a need for a method or system to improve the contrast ratio and reduce the cost of vertically aligned LCDs.